A variable-resistance circuit, such as a variable attenuator, can be used to lower voltage, dissipate power and/or improve resistance matching in radio frequency (RF) communication systems.
In receivers, such as those used in various portable electronic devices and/or base stations, it is often necessary to attenuate large received signals before providing the attenuated signal to more sensitive receiver components for proper signal processing. Additionally, in transmitters, such as those used in various portable electronic devices and/or base stations, it is often useful to control the transmitted signal to have less power, in order to avoid excessive interference with other equipment.
In most receivers and transmitters, it is generally desirable to have high linearity and low distortion, particularly over a wide range of signals. Hence, these characteristics are important when designing a variable attenuator for a communication system in which signals are transmitted and received over a large range of power levels.
However, the performance of a variable attenuator may degrade significantly over a high dynamic attenuation range, for example exhibiting a high degree of distortion in the RF communication system. Reasons for such distortion include error in resistance matching between the RF attenuation circuits and DC control circuits that control the amount of variable attenuation. Other than attenuation circuits, other RF circuits may also suffer from such resistance mismatch. For example, phase shifters and variable gain amplifiers are also RF circuits that require control circuits to closely control operation.
Accordingly, solutions for providing an improved variable attenuator are desirable. Further, it would be desirable to provide a control circuit that is suitable for controlling operation of RF circuits, such as attenuation circuits, phase shifters, vector modulators and variable gain amplifiers, among others.